1. Field of the Invention
The present invention relates to a technique for forming an image based on digital image data on a printing medium to have high quality.
2. Description of the Related Art
In recent years, digital image data captured by a digital camera is normally printed by a printer. The number of tones of image data which can be printed by a printer is generally smaller than that of image data to be handled by a digital camera and PC. On the other hand, the resolution of an image formed by the printer is generally higher than that of an image displayed by a display device such as a PC monitor. Hence, upon printing digital image data by a printer, the number of tones of the digital image data is efficiently decreased by halftone processing. The halftone-processed image data further undergoes image processing in correspondence with a type of a printer which executes printing, and is then printed on a printing medium such as a paper sheet. Note that the type of the printer includes, for example, an ink-jet printer and laser beam printer.
In a print operation by a printer, positions of dots to be printed on a printing medium often fluctuate due to a conveyance error of a printing medium and the like. For example, in an ink-jet printer, an error is often generated between an ideal conveyance amount and actual conveyance amount of a printing medium due to decentration of a motor, slippage of a printing medium, and the like. Such error often causes stripe-like density fluctuations at joint portions between respective printing scans, and an degradation of sharpness. In a laser beam printer, toner images of respective colors are superposed while being deviated from their ideal positions due to various causes such as vibrations of a printer main body, part accuracy variations of respective mechanisms, and the like, thus causing color misalignment at an edge portion of an image.
A technique for eliminating image quality deterioration caused by such position fluctuation of dots to be printed is available. For example, Japanese Patent Laid-Open No. 04-369158 discloses a technique for adjusting a position of a printer head by detecting an error of a printing medium conveyance amount in an ink-jet printer. Also, Japanese Patent Laid-Open No. 01-183676 discloses a technique for controlling the irradiation timing of each laser beam and a position of each reflection mirror based a position shift amount detected using a color misalignment detection mark in a laser beam printer. On the other hand, Japanese Patent Laid-Open No. 2009-262457 discloses a technique for obtaining density information of an image already printed on a printing medium and feeding it back to an input image in an ink-jet printer. This technique can obtain high image quality even when the number of divided passes is reduced in a multi-pass printing technique of the ink jet printer, and can also improve a print speed.
However, the technique disclosed in Japanese Patent Laid-Open No. 04-369158 requires a new adjusting mechanism for positional adjustment of the printer head. In order to attain accurate position correction, the adjusting mechanism requires parts which have higher accuracy than at least a printing medium conveyance mechanism, resulting in an increase in cost of a printer product. Likewise, the technique disclosed in Japanese Patent Laid-Open No. 01-183676 has to use high-cost parts so as to attain accurate position correction. Also, since the techniques disclosed in Japanese Patent Laid-Open Nos. 04-369158 and 01-183676 require mechanical control of the adjusting mechanism, correction results are often not stable due to the influences of the weight of the mechanism itself and inertia (inertia moment). Furthermore, the method disclosed in Japanese Patent Laid-Open No. 2009-262457 can improve density fluctuations, but cannot correct position shifts, thus worsening edge sharpness.